[clang][AST] Improve diagnostic for `nullptr` constexpr function pointer call

This patch improves diagnostic for clang constexpr evaluator by adding
a check for nullptr in function pointer call evaluations.

ex.
```
constexpr int foo(int (*bla)(void)) {
  return bla();
}

static_assert(foo(nullptr) == 1);
```

BEFORE this patch, clang generates the following diagnostic for the
code above:

```
<source>:5:15: error: static assertion expression is not an integral constant expression
static_assert(foo(nullptr) == 1);
              ^~~~~~~~~~~~~~~~~
<source>:2:10: note: subexpression not valid in a constant expression
  return bla();
         ^
<source>:5:15: note: in call to 'foo(nullptr)'
static_assert(foo(nullptr) == 1);
              ^
1 error generated.
```

AFTER this patch, subexpression not valid in a constant expression note
is replaced with 'bla' evaluates to a null function pointer.

Fixes https://github.com/llvm/llvm-project/issues/59872
Differential Revision: https://reviews.llvm.org/D145793